Ignore "line too long" errors from flake8 on db migrations

Black reformats these files before flake8 lints them, but:
* it doesn't (yet) wrap the long strings (see psf/black#182);
* it has issues with inserting trailing commas on nested collections, regularly resulting in causing pycodestyle E231 errors.

A case could be made for excluding db migrations from linting altogether, tbh.

.flake8

@@ -1,9 +1,14 @@
 [flake8]
+# B950  line too long
+# E231  missing whitespace after ‘,’
 # E266  too many leading ‘#’ for block comment
 # E303  too many blank lines (3)
+# E501  line too long
 # F403  ‘from module import *’ used; unable to detect undefined names
 # W503  line break before binary operator
 ignore = E266, E303, F403, W503
 max-line-length = 89
 max-complexity = 18
 select = B,C,E,F,T,W,B9
+per-file-ignores =
+    */migrations/*:B950, E231, E501